Multicrystalline silicon substrate and process for roughening surface thereof

ABSTRACT

A surface of a multicrystalline silicon substrate is etched with an alkaline aqueous solution in a condition so that a surface area-to-planar surface area ratio R is smaller than 1.1. A multiplicity of fine textures are formed over the irregularities by dry etching. This allows fine textures to be formed uniformly, and a solar cell with high efficiency can thus be produced.

This application is based on application No. 2003-019535 filed in Japan,the content of which is incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multicrystalline silicone substrateand a process for roughening surface thereof, preferably used in a fieldof solar cell or the like.

2. Description of the Related Art

Solar cell is a device that converts light energy incident on itssurface such as sunlight into electric energy. Various approaches havebeen attempted in order to improve efficiency in converting light energyinto electric energy. One of such approaches is technique that reducesreflectance of light incident on the surface of the substrate. Reducingthe reflectance of light incident on the surface allows the conversionefficiency into electric energy to be improved.

The main types of solar cells are classified by material used intocrystalline silicon solar cells, amorphous silicon solar cells, compoundbased solar cells and the like. Most of the solar cells that aredistributed in the market are crystalline silicon solar cells. Thecrystalline silicon solar cells are further classified into singlecrystal type and multicrystalline type. Single crystal silicon solarcells have the advantage that the conversion efficiency is relativelyhigher because of the high quality of the substrates. However, they havethe disadvantage of high production cost of the substrates. In contrast,multicrystalline silicon solar cells have the disadvantage of inferiorsubstrate quality making it difficult to improve conversion efficiency,while they have the advantage of low production cost. In addition, as aresult of recent improvement in substrate quality of multicrystallinesilicon solar cells and advancement of cell fabrication technology,conversion efficiency on the order of 18% has been achieved atlaboratory level for multicrystalline silicon solar cells.

Meanwhile, multicrystalline silicon solar cells at mass-production levelhave been hitherto on the market because of their low cost. The demandhas recently been further increasing under circumstances whereenvironmental issues are of great concern. Accordingly, they arerequired to have higher conversion efficiency in addition to thelow-cost benefit.

In fabricating a solar cell using a silicon substrate, etching a surfaceof the substrate in a predetermined condition with an alkaline aqueoussolution such as a sodium hydroxide solution results in formation oftextures on the surface, which reduces reflection of light at thesurface to some extent.

When a single crystal silicon substrate with (100) orientation is used,a pyramidal pattern called textured structure can be formed uniformly onthe surface of the substrate by such a process.

However, when fabricating a solar cell using a multicrystallinesubstrate, since texture etching with an alkaline aqueous solutiondepends on the crystal orientation, such a pyramidal pattern cannot beformed uniformly. For this reason, this poses the problem that theoverall reflectance cannot be reduced effectively.

In order to overcome such a problem, preparing a surface having finetextures by means of Reactive Ion Etching has been proposed for the casewhere a solar cell is fabricated using a multicrystalline siliconsubstrate (for example, in the patent document [1]). By this method,fine textures can be uniformly formed independent of the irregularorientation of the crystals of multicrystalline silicon. In particular,reflectance can be more effectively reduced in solar cell usingmulticrystalline silicon.

[1] Japanese Patent Laid-Open Publication No. 1997-102625

Multicrystalline silicon substrates are generally fabricated by slicinga block or ingot of silicon obtained by a casting process. Widely usedis slicing with use of an ID blade or multi-wire saw. Silicon substratesfabricated by such a process include a layer that is mechanicallydamaged by the slicing remaining on the surface, which causesdegradation of the solar cell performance. The damaged layer needs to beremoved in order to use the substrate for a solar cell. The thickness ofthe damaged layer is, although it varies depending on the machiningprocess, generally on the order of 10 μm.

However, since the required depth to which the surface is etched by theforegoing reactive ion etching for forming fine textures is severalmicrons at the largest, the damaged layer is hardly removed.

In order to form fine textures on the surface of a substrate for solarcell by reactive ion etching, the damaged layer is preferably removedprior to the formation.

Except for mechanical etching, various processes can be employed for theremoval of the damaged layer. Although any of them may be used, inparticular, wet etching with chemicals is a simple and easy process. Ingeneral, the easiest and inexpensive process is considered to be anetching with use of an alkaline aqueous solution including sodiumhydroxide or potassium hydroxide.

However, it has been revealed that removal of the damaged layer byalkaline etching prior to reactive ion etching for forming fine texturessometimes results in formation of too complex irregularities on thesubstrate, and that the performance of solar cells fabricated using suchsubstrates are degraded.

The object of the present invention is to provide a multicrystallinesilicon substrate and a process for roughening a surface thereof thateffectively improves the solar cell performance even in a condition ofremoval of the damaged layer by alkaline etching prior to reactive ionetching for forming fine textures.

A multicrystalline silicon substrate according to the present inventioncomprises: a substrate of multicrystalline silicon having relativelylarge irregularities formed on a surface thereof by etching with analkaline aqueous solution; and a multiplicity of relatively finetextures formed over the relatively large irregularities by dry etching,wherein a ratio r expressed as r=a/b, which is the ratio between thelength a of a virtual line connecting individual peaks of the relativelyfine textures at a vertical cross section thereof and the length b of astraight line connecting the endpoints of the virtual line, is notsmaller than 1 and smaller than 1.1.

This multicrystalline silicon substrate allows fine textures to beformed uniformly with even heights, thereby effectively reducingreflectance in a solar cell fabricated using the substrate. Solar cellswith high conversion efficiency can thus be fabricated.

It is preferable that the fine textures have a height and a width of 2μm or less, respectively. More preferably, the fine textures have aheight and a width of 1 μm or less, respectively.

Preferably, the fine textures have a height-to-width aspect ratio of 2or less. At aspect ratios greater than 2, the fine textures may sufferbreakage during the production process causing large leak current tooccur in the fabricated solar cell, which therefore fails to have goodoutput performance.

A process for roughening a surface of a solar cell substrate accordingto the present invention comprises the steps of: etching a surface of amulticrystalline silicon substrate with an alkaline aqueous solution forforming relatively large irregularities having a surface area-to-planarsurface area ratio R of larger than 1 and smaller than 1.1; and a dryetching step for forming a multiplicity of relatively fine textures overthe relatively large irregularities.

This process allows fine textures to be formed uniformly, therebyeffectively reducing reflectance in the solar cell fabricated using thesubstrate. Solar cells with high conversion efficiency can thus befabricated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a solar cell using a multicrystalline siliconsubstrate that is surface textured by a process of the presentinvention.

FIG. 2 illustrates a reactive ion etching device used in a process forroughening a surface of a multicrystalline silicon substrate accordingto the present invention.

FIG. 3 is a SEM image of a surface of a multicrystalline siliconsubstrate textured by a surface texturing process of the presentinvention.

FIG. 4 is a diagram illustrating the definitions of surface area andplanar surface area according to the present invention.

FIG. 5 is a graph showing a relationship between surface area-to-planarsurface area ratio R after alkaline etching and conversion efficiency ofthe fabricated solar cell.

FIG. 6 is a schematic cross-sectional view showing fine textures 22formed by reactive ion etching over a surface of a silicon substrate 21after removal of a damaged layer by alkaline etching.

FIG. 7 is an electron microscope image for illustrating aspect ratio inthe case of fine textures formed over a surface of a silicon substrate.

DETAILED DESCRIPTION OF THE INVENTION

A specific embodiment of the present invention will be hereinafterdescribed in detail with reference to the accompanying drawings taking abulk crystalline silicon solar cell as an example. However, the presentinvention is not limited to this example and applicable to solar cellsof other types including thin film solar cells using a glass substrate.

FIG. 1 is a cross section showing a structure of a solar cell fabricatedby a substrate processing method of the invention. Referring to FIG. 1,numeral 1 denotes a silicon substrate, numeral 2 denotes textures formedon the silicon substrate 1, numeral 3 denotes an impurity diffusinglayer on the light-reception surface side, numeral 4 denotes an impuritydiffusing layer (BSF) on the back surface side of the silicon, numeral 6denotes a surface electrode, and numeral 7 denotes a back surfaceelectrode.

On the surface side of the silicon substrate 1, impurity diffusing layer3 in which another type impurity against the substrate is diffused isformed. This impurity diffusing layer 3 is provided for forming asemiconductor junction within the silicon substrate 1. For example, fordiffusion of an n-type impurity, a vapor phase diffusion technique usingPOCl₃, spin-on diffusion using P₂O₅ or ion implantation for directlyintroducing P⁺ ions into the substrate by electric field is used.

The layer 3 including an opposite-type semiconductor impurity is formedto a thickness of 0.3–0.5 μm.

An antireflective film 5 is formed on the surface side of the siliconsubstrate 1. The antireflective film 5 is provided to prevent light fromreflecting at the surface of the silicon substrate 1 so as toeffectively introduce light into the silicon substrate 1. As a result oftaking into account of the difference in index of refraction between thesilicon substrate 1 and the antireflective film and the like, theantireflective film 5 comprises a material having an index of refractionon the order of 2, and is a silicon nitride film or silicon oxide (SiO₂)film with a thickness of 500–2000 Å.

Preferably, a layer 4 doped with a same-type semiconductor impurity asthat of substrate at a high concentration is formed on the backsidesurface of the silicon substrate 1. This layer is called Back SurfaceField (BSF) layer. The BSF layer 4 doped with a first-type semiconductorimpurity at a high concentration is provided to create an internalelectric field on the backside of the silicon substrate 1 so as toprevent recombination of carriers in the vicinity of the backsidesurface of the silicon substrate 1, thereby preventing lowering of theefficiency.

In the above described structure of the solar cell, carriers generatedin the vicinity of the back surface of the silicon substrate 1 areaccelerated by the electric field, and as a result, electric power canbe generated effectively. In particular, photosensitivity to light atlong wavelengths is enhanced, so that degradation of the solar cellperformance at high temperatures can be alleviated. The sheet resistanceon the backside of the silicon substrate 1 formed with the BSF layer 4is on the order of 15 Ω/sq.

A surface electrode 6 and a back surface electrode 7 are formed on thesurface side and backside of the silicon substrate 1, respectively.These surface electrode and back surface electrode are formed by screenprinting of Ag paste and thereafter baking Ag paste mainly composed ofAg powder, a binder and frit, and then by forming a solder layerthereon.

The surface electrode 6 is constructed with a large number of fingers(not shown) formed at pitches of about 3 mm across a width of about 200μm, and two busbars interconnecting the large number of fingers.

The back surface electrode 7 is constructed with a large number offingers (not shown) formed at pitches of about 5 mm across a width ofabout 300 μm, and two busbars interconnecting the large number offingers.

The silicon substrate 1 is a multicrystalline silicon substrate. Thissubstrate 1 may either be of p-type or n-type. The multicrystallinesilicon substrate 1 is formed by the casting method or the like. Beingmass-producible, multicrystalline silicon is quite advantageous oversingle crystal silicon in terms of production cost. A silicon substrateis produced by cutting a silicon block formed by a crystal-pullingmethod or casting method into ingots of 10 cm×10 cm or 15 cm×15 cm insize, and then slicing the ingot into pieces of about 300 μm thickness.

A typical method used for slicing the ingot is sawing with an ID (InsideDiameter) blade or multi-wire saw. Because silicon is sliced by such amechanical method, residual stress is present on the ground surface,which therefore arises a number of defects in the vicinity of thesurface of the silicon substrates. Accordingly, in the manufacture ofmulticrystalline silicon solar cells, the damaged layer as above needsto be removed since it causes solar cell performance degradation.

In order to remove the damaged layer, a process that causes noadditional damage to the surface of the silicon substrate is necessary.Wet etching is generally used as such a process.

The most frequently used process is etching with sodium hydroxide orpotassium hydroxide because of the ease and inexpensiveness. The etchingwith alkaline aqueous solution has a selectivity depending on thecrystallographic orientation selectivity preference when etchingmulticrystalline silicon.

Among the multicrystalline silicon grains, grains of (100) orientationare etched at the fastest etching rate, while grains of (111)orientation are etched at the slowest etching rate. The preference inthe etching rate is also effected among individual crystal grains withinthe multicrystalline silicon substrate so that the etching proceeds toreveal (111) orientation planes ultimately.

As a process for forming irregularities utilizing the foregoing feature,there is a process for forming a textured surface with use of analkaline aqueous solution. In this process, etching is performed, forexample, with a 5% sodium hydroxide aqueous solution at about 70° C. Ifthe crystal grains are of (100) orientation, a myriad of pyramidalstructures having four (111) planes are formed on the surfaces of thecrystal grains. Accordingly, in cases where a single crystal siliconsubstrate of (100) orientation is used, pyramidal structures are formedon the whole surface of the single crystal silicon substrate. Wherethere are the pyramidal structures formed, since they function to reducereflection of light, the light absorption is increased, therebyimproving the solar cell performance. For this reason, siliconsubstrates of (100) orientation are frequently used for single crystalsilicon solar cells.

In the case of a multicrystalline silicon substrate, pyramidalstructures are formed on the (100) oriented planes by alkaline etching.While other planes are not formed with pyramidal structures, since theetching proceeds so as to reveal the (111) planes, textures are formedcorrespondingly. If etched to a depth of about 15 μm with alkalinesolution, the sizes of the textures formed by alkaline etching are aboutseveral μm to less than 20 μm.

In the present invention, a reactive ion etching (RIE) process isadditionally performed after the removal of the damaged layer with analkaline aqueous solution, so as to form fine textures. This is becauseit is impossible to remove the damaged layer effectively by RIE process.The RIE process etches only several microns at most when forming finetextures on all the surfaces of the crystal grains of differentorientations present on the surface of the multicrystalline siliconsubstrate.

The RIE process is an etching process, in which gases are introducedinto an evacuated chamber, and while the pressure is kept constant, a RFvoltage is applied to an electrode situated in the chamber so that aplasma is generated, which yields active species such as ions andradicals that act on the surface of the substrate, thereby etching thesurface.

This process is generally carried out using a device as shown in FIG. 2.

In FIG. 2, there are shown a mass-flow controller 8, a silicon substrate1, an RF electrode 9, a pressure regulator 10, a vacuum pump 11, and anRF power source 12. Gases are introduced into the chamber from themass-flow controller 8. A plasma is generated at the RF electrode sothat ions and radicals are excited and activated to act on the surfaceof the silicon substrate 1 placed above the RF electrode 9, therebyetching the substrate.

In etching processes where active species are produced, the method inwhich the effect of the ions on the etching is particularly enhanced isgenerally called reactive ion etching. A process similar to this isplasma etching, where the principle of plasma generation is basicallythe same. The reactive ion etching differs from the plasma etching onlyin that the distribution of the active species acting on the substrateis altered to a different one by the arrangement of the chamber,electrode or frequencies generated, and is the same as the plasmaetching in other aspects. Accordingly, the present invention applies notonly to reactive ion etching, but more widely to plasma etching ingeneral.

In the present invention, for example, with a Cl₂ gas flow of 0.01 slm,an O₂ gas flow of 0.06 slm and a SF₆ gas flow of 0.04 slm, etching isperformed under a reaction pressure of 7 Pa and at an RF power of 5 kWfor generating plasma for about 5 minutes. By this process, finetextures are formed on a surface of the silicon substrate.

Although silicon is etched and essentially vaporized during the etching,apart thereof is not vaporized and molecules coalesce each other toremain on the surface of the substrate as a residue. The main componentof the residue is silicon.

If the conditions of the gases, reaction pressure, RF power and the likeare set so that fine textures are formed and the residue as mentionedabove remains on the surface of the substrate, fine textures are formedby etching that proceeds utilizing the residue as a mask. Accordingly,textures can be formed reliably. However, as described later, the aspectratio of the fine textures needs to be optimized by satisfying aspecific condition.

FIG. 3 shows a surface where fine textures are formed by reactive ionetching after removal of damaged layer by etching with an alkalineaqueous solution. The lower photograph of FIG. 3 is enlarged one of theupper photograph.

The sizes of the fine textures formed by reactive ion etching are about0.1–1.0 μm, which are one order of magnitude smaller than the texturesof the texture formed by etching with an alkaline aqueous solution.

In addition, since the formation of textures poorly depends on thecrystal orientation in reactive ion etching, a multicrystalline siliconsubstrate has been texture etched to form uniform fine textures withreactive ion etching after removal of the damaged layer with use of thealkaline aqueous solution.

FIG. 4 is a schematic diagram showing a random irregularities formed ona surface of a multicrystalline silicon substrate with use of analkaline aqueous solution. Some irregularities are drawn as a group ofsmall triangles A to J.

In this specification, an area in which the areas of textures of thesurface of the silicon substrate are included (in FIG. 4, the sum of theareas of the triangles: A+B+C+D+E+F+G+H+I+J) is referred to as the“surface area”, and a projected area that is the area viewed from adirection perpendicular to the silicon substrate (S in FIG. 4) isreferred to as the “planar surface area”. The surface area can bemeasured with AFM (Atomic Force Microscope) or SEM (Scanning ElectronMicroscope) for three-dimensional surface measurements.

Irregularities formed by alkaline etching have surface area-to-planarsurface area ratios R of about 1.1 to 1.3. By definition, the ratio R isnever smaller than 1.

When fine textures are further formed over the irregularities byreactive ion etching, the surface area formed by etching ofirregularities with alkaline aqueous solution further increases. As thesurface area increases, difference due to the different crystalorientations becomes prominent. It has been found that, for this reason,when an antireflective film is formed by the CVD method in a laterprocess, color unevenness is caused by the film thicknesses that differfrom crystal grain to crystal grain, leading to degradation of the solarcell performance. Also in view of the solar cell performance, a smallersurface area is preferable, because as the surface area increases,saturation current increases leading to an increase in the leakage ofthe current. In other words, the surface of the silicon substrate beforethe formation of fine textures by reactive ion etching is preferably asflat as possible.

After a number of experiments, it has been discovered that in order toeffectively form fine textures by reactive ion etching after removal ofthe damaged layer on the surface of a multicrystalline silicon substrateby alkaline etching, it is preferable that the surface area-to-planarsurface area ratio R after the removal of the damaged layer by alkalineetching be smaller than 1.1.

By this arrangement, the thickness of the antireflective film can bemaintained almost constant irrespective of the crystal grains on thesurface of the substrate, so that the performance can be improvedsufficiently.

FIG. 5 is a graph showing a relationship between surface area-to-planarsurface area ratio R of a substrate after removal of a damaged layer byalkaline etching and conversion efficiency of a solar cell fabricated byforming fine textures by reactive ion etching using the substrate. Thegraph shows that high conversion efficiencies are achieved when thesurface area-to-planar surface area ratio R after removal of the damagedlayer by alkaline etching is smaller than 1.1.

FIG. 6 is a cross-sectional view showing fine textures 22 formed on asurface of a silicon substrate after removal of a damaged layer byalkaline etching.

The present invention is arranged such that ratio r (r=a/b) between thelength a of a virtual line (the bold line in FIG. 6) connectingindividual peaks of the fine textures 22 at a cross section thereof andthe length b of a straight line connecting the two endpoints 23 and 24of the virtual line is smaller than 1.1, that is r<1.1. This indicatesthat the heights of the peaks of the fine textures 22 are even.Meanwhile, by definition, the ratio r is never smaller than 1.

In order that the ratio R between the surface area and planar surfacearea after removal of the damaged layer by alkaline etching is smallerthan 1.1, and/or the ratio r between the length a of a virtual lineconnecting individual peaks of the fine textures 22 at a cross sectionthereof and the length b of a straight line connecting the two endpointsof the virtual line is smaller than 1.1, performing removal of thedamaged layer with the alkaline etching as fast as possible would beeffective.

This can be accomplished, for example, by a process in which a 25 wt %sodium hydroxide aqueous solution is heated at 85° C., and a siliconsubstrate is etched in the solution. In the present invention, thefeatures that ratio R between surface area and planar surface area issmaller than 1.1, and ratio r between the length a of a virtual lineconnecting individual peaks of the fine textures at a cross sectionthereof and the length b of a straight line connecting the two endpointsof the virtual line is smaller than 1.1 can be accomplished byperforming the alkaline etching at a high speed.

As shown in FIGS. 6 and 7, the fine textures form conical shapes orstrings of conical shapes. The size thereof can be changed by the gasconcentration or etching time in the RIE process.

The fine textures are formed to have a width and a height of 2 μm orless, respectively. In order to controllably form fine textures so as touniformly and precisely cover all over the required area of the surfaceof the silicon substrate 1, the width and height are preferably 1 μm orless, respectively.

The aspect ratio used for describing the present invention is defined,as shown in FIG. 7, as the ratio between the height and the base of atriangle that is formed by the generally straight lines of the bothsides observed at a cross-section of the fine textures (height/width ofan texture). Where the cones of the fine textures form a string, thecross-section of the fine textures mentioned above refers to a verticalcross-section taken along the string.

Since the height and width of the fine texture in FIG. 7 are 1.3 μm and1 μm, respectively, the aspect ratio is 1.3.

In the present invention, the aspect ratio of the fine textures ispreferably 2 or less. When the aspect ratio is greater than 2, breakageof the fine textures occurs in the manufacturing process, and leakagecurrent is great in the fabricated solar cell, which therefore fails tohave good output performance.

While a specific embodiment of the present invention has been describedso far, implementation of the present invention is not limited to theforegoing embodiment, and various modifications may be made within thescope of the present invention.

1. A multicrystalline silicon substrate comprising: a substrate ofmulticrystalline silicon having irregularities formed on a surfacethereof; and a multiplicity of textures formed on the irregularities,wherein the textures are smaller than the irregularities, wherein aratio r expressed as r=a/b, which is the ratio between the length a of avirtual line connecting individual peaks of the textures at a verticalcross section thereof and the length b of a straight line connecting theendpoints of the virtual line, is equal to or larger than 1 and smallerthan 1.1.
 2. The multicrystalline silicon substrate according to claim1, wherein the fine textures have a height and a width of 2 μm or less,respectively.
 3. The multicrystalline silicon substrate according toclaim 1, wherein the fine textures have a height and a width of 1 μm orless, respectively.
 4. The multicrystalline silicon substrate accordingto claim 1, wherein the textures have a height-to-width aspect ratio(height/width) of 2 or less.
 5. The multicrystalline silicon substrateaccording to claim 1, wherein the irregularities are formed by etchingwith an alkaline aqueous solution.
 6. The multicrystalline siliconsubstrate according to claim 1, wherein the textures are formed by dryetching.
 7. A multicrystalline silicon substrate comprising: a substrateof multicrystalline silicon having irregularities formed on a surfacethereof; and a surface area-to-planar surface area ratio R of thesubstrate being larger than 1 and smaller than 1.1, wherein amultiplicity of textures are formed on the irregularities, and whereinthe textures are smaller than the irregularities.
 8. Themulticrystalline silicon substrate according to claim 7, wherein a ratior expressed as r=a/b, which is the ratio between the length a of avirtual line connecting individual peaks of the textures at a verticalcross section thereof and the length b of a straight line connecting theendpoints of the virtual line, is equal to or larger than 1 and smallerthan 1.1.
 9. The multicrystalline silicon substrate according to claim7, wherein the irregularities are formed by etching with an alkalineaqueous solution.
 10. The multicrystalline silicon substrate accordingto claim 7, wherein the textures are formed by dry etching.